1. Field of the Invention
The present invention relates to an improved sensor unit that measures a physical amount in a process control. The present invention is efficiently applied to a sensor unit for a differential pressure/pressure transmitter, and more particularly to a pressure container, and a vacuum sealed device, a vacuum sealed element or an assembly thereof.
Priority is claimed on Japanese Patent Application No. 2011-139384, filed Jun. 23, 2011, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
FIG. 9 is a cross-sectional view illustrating a configuration example of a sensor unit 1 applied to a differential pressure/pressure transmitter in accordance with the related art. A vibratory sensor part 10 is fixed on a support 20 formed of an insulator having a hollow part. A metal body 30 includes a first face 30a and a second face 30b opposite to each other with a predetermined interval. The support 20 is mounted on the first face 30a of the metal body 30. A structure and operation principle of a vibratory differential-pressure sensor are disclosed in detail in Japanese Examined Patent Application, Second Publication No. H8-10169 and NIKKEI ELECTRONICS 1988 Jun. 27 (No. 450) pp 96-97.
The metal body 30 includes a plurality of through-holes 30c formed in a thickness direction thereof. A plurality of metal pins 40, which are inserted from the second face 30b into the through-holes 30c of the metal body 30, pass through the support 20, and are electrically connected to a predetermined terminal of the sensor part 10. The metal pins 40 serve as input and output terminals of the sensor part 10.
Each through-hole 30c of the metal body 30 has an internal hermetic structure in which the corresponding metal pin 40 and the metal body 30 are sealed by a glass material 50. The glass material 50 includes concave menisci 50a on upper and lower surfaces thereof.
An upper portion of the sensor part 10 is covered with a sealing liquid capsule 60, and includes a sealing liquid chamber L. The sealing liquid capsule 60 is covered with a cup-shaped metal cap 70, and an open end thereof is welded to a circumferential end of the first face 30a of the metal body 30.
FIG. 10 is a cross-sectional view illustrating a junction relation of the sensor unit to a housing in accordance with the related art. The sensor unit 1 shown in FIG. 10 is inserted into a hole 80a of the housing 80 in a direction P, and a circumferential end of the second face 30b of the metal body 30 is fixedly joined with a circumferential end of the hole 80a of the housing 80 by welding 90. The housing 80 is connected to a frame ground FG.
The case in which noise is applied to the frame ground FG will be described using the configuration shown in FIGS. 9 and 10. The noise applied to the frame ground is input to the metal pins 40 via parasitic capacitance of the portion sealed by the glass material 50, and is transferred to a signal line.
FIGS. 11A and 11B are diagrams illustrating an equivalent circuit formed of noise, parasitic capacitance, and a sensor part in accordance with the related art. FIG. 11A shows an equivalent circuit when noise Vni is applied to the sensor part 10 made up of an H-type oscillator driven electromagnetically. Among the symbols, Ch1 and Ch2 indicate capacitances between the metal pins 40 and the frame ground FG, R1 to R3 indicate resistances of the oscillator, and Vno indicates an output noise voltage. The output noise voltage Vno is input into a differential amplifier Q.
FIG. 11B is an equivalent circuit into which that of FIG. 11A is simplified. In this equivalent circuit, a transfer characteristic of noise G(s) is expressed as Equation 1.
                              G          ⁡                      (            s            )                          =                                            V              no                                      V                              n                ⁢                                                                  ⁢                1                                              =                                                    (                                                                            1                                              sC                                                  h                          ⁢                                                                                                          ⁢                          2                                                                                      ⁢                                          R                      1                                                        -                                                            1                                              sC                                                  h                          ⁢                                                                                                          ⁢                          1                                                                                      ⁢                                          R                      2                                                                      )                            ⁢                              R                3                                                                                                                                                        1                                                  sC                                                      h                            ⁢                                                                                                                  ⁢                            2                                                                                              ⁢                                                                        R                          2                                                ⁡                                                  (                                                                                    1                                                              sC                                                                  h                                  ⁢                                                                                                                                          ⁢                                  1                                                                                                                      +                                                          R                              1                                                                                )                                                                                      +                                                                  1                                                  sC                                                      h                            ⁢                                                                                                                  ⁢                            1                                                                                              ⁢                                                                        R                          1                                                ⁡                                                  (                                                                                    1                                                              sC                                                                  h                                  ⁢                                                                                                                                          ⁢                                  2                                                                                                                      +                                                          R                              2                                                                                )                                                                                      +                                                                                                                                          (                                                                        1                                                      sC                                                          h                              ⁢                                                                                                                          ⁢                              2                                                                                                      +                                                  R                          2                                                                    )                                        ⁢                                          (                                                                        R                          1                                                +                                                  1                                                      sC                                                          h                              ⁢                                                                                                                          ⁢                              1                                                                                                                          )                                        ⁢                                          R                      3                                                                                                                              Equation        ⁢                                  ⁢        1            
It can be seen from FIG. 11B that, since the equivalent circuit is a bridge circuit, noise is easily transferred when variability between R1 and R2 and variability between Ch1 and Ch2 increase. Since R1 and R2 are formed by a semiconductor process, the variability can be reduced.
On the other hand, the variability between Ch1 and Ch2 is dependent on mechanical precision, and thus tends to increase, compared to the variability between R1 and R2. The parasitic capacitance Ch between the frame ground FR and the metal pin 40 is expressed as Equation 2.
                              C          h                =                                            2              ⁢              πɛ                                      ln              ⁡                              (                                  b                  a                                )                                              ⁢          L                                    Equation        ⁢                                  ⁢        2            
To secure a breakdown voltage structure, a ratio b/a of a radius b of the through-hole 30c to a radius a of the metal pin 40 in the portion sealed by the glass material 50 is set to a range between 2 and 3.
The parasitic capacitance between the metal pin 40 (i.e., signal line) and the frame ground FG has three variabilities: a sealing radius ratio b/a, a sealing length L, and sealing eccentricity.
FIGS. 12A to 12F are characteristic diagrams that explain a relation between the sealed portion and the parasitic capacitance in accordance with the related art. FIG. 12A shows a parasitic capacitance variability according to the sealing radius ratio b/a, and FIG. 12D shows a sensitivity variability according to the sealing radius ratio b/a. FIG. 12B shows a parasitic capacitance variability according to the sealing length L, and FIG. 12E shows a sensitivity variability according to the sealing length L. FIG. 12C shows a parasitic capacitance variability according to the sealing eccentricity, and FIG. 12F shows a sensitivity variability according to the sealing eccentricity.
FIG. 13 is a table illustrating measured values of parasitic capacitance, in which the values of FIGS. 12A to 12F are arranged in accordance with the related art. When a relative permittivity c of the glass material is set to ε=7, a hole radius b of the glass insertion part of the metal body is set to b=3 mm, a radius a of the metal pin is set to a=1 mm, and a sealing length L is set to L=7 mm as the parameters, the parasitic capacitance Ch calculated according to Equation 2 is Ch=2.5 pF.
In the variability, a measured value is equal to a calculated value. In the difference in average value, the sealing radius ratio falls within a range of the variability, because the dimensions can be controlled. The sealing eccentricity is negligible, because the sensitivity is low. In the sealing length L, since the menisci 50a are formed at the sealed portion, the dimensions cannot be controlled with high precision. This is a major factor of the variability.
FIG. 14 is a characteristic diagram illustrating results of calculating a frequency characteristic of the transfer characteristic G(s) of noise obtained by substituting variability ΔC of the parasitic capacitance into Equation 1 in a configuration example in accordance with the related art. As shown in FIG. 14, it can be found that, as ΔC increases, the transfer characteristic G(s) is easily influenced by the noise.